Have you tried changing the language and then deleting the BitRaider folder in you SWTOR installation?

It looks like the language changing might have corrupted something in your install, and deleting that folder forces the client to fully repair itself the next time you play.

If that doesn't work, see if you can find the language settings within the files for the install. That might allow you to change the language at the install-level instead of through the launcher. I'm not at home right now, so I couldn't begin to tell you where to look, but it has to be there somewhere.

The only other option that I can think of would be a clean install to see if the issue persists, and if it does then email SWTOR support/create a ticket.